This invention relates to a radio receiver circuit comprising, in cascade, a bandpass filter for a desired received signal, a frequency translator circuit for translating the desired received signal to an intermediate frequency, and a demodulator circuit for demodulating the frequency-translated desired received signal, said frequency translator circuit comprising a local oscillator circuit and a mixer circuit to a first input of which the output of the local oscillator circuit is coupled and to a second input of which the output of the bandpass filter is coupled. The receiver circuit includes a tuning control circuit for the bandpass filter, which tuning control circuit comprises means for applying to the input of the bandpass filter a locally generated signal the frequency of which is substantially equal to that of the desired received signal and a control loop coupling an output of the frequency translator circuit to a tuning control input of the bandpass filter, for automatically tuning the bandpass filter to substantially the frequency of said locally generated signal.
One known arrangement of this general kind is disclosed in Japanese Kokai No. 59-027613. In this known arrangement the output frequency of the local oscillator circuit is variable to thereby tune the arrangement to different desired received signals, and the bandpass filter is tuned simultaneously. The purpose of the tuning control circuit is to eliminate tracking errors between the two. To ensure that the tuning control circuit operates satisfactorily even when the desired received signal is weak the input of the bandpass filter is switched temporarily from an antenna to the output of an auxiliary circuit which locally produces a "pseudo broadcast signal".
It is often an aim of electronic circuit design and deveopment to enable as much as possible of the circuit to be integrated on one or more semiconductor chips to minimize the assembly operations required to implement an operating system and/or to enable the volume occupied by the circuit to be reduced. Such chips are normally encapsulated for protective reasons, the encapsulations being provided with external conductive connectors to enable parts of the integrated circuit to be electrically accessed from the exterior, for example for power supply and signal input and output purposes. The connectors are normally conductively connected to the relevant points of the chip by so-called bond wires. One circuit element which it is not easy to implement in integrated circuit form is an inductor, and this has led to inductance being simulated by such things as capacitively terminated gyrator circuits. However these circuits themselves are impracticable when they are required to operate at frequencies in the order of, for example, 10.sup.8 or 10.sup.9 Hz, which frequencies are becoming increasingly of interest for radio purposes. Thus, for example, the inductive component of an RF bandpass filter for a radio receiver operating in a frequency band in the 10.sup.8 or 10.sup.9 Hz range has normally to be implemented as a discrete inductor, which fact is on the face of it in conflict with the above-mentioned aims of minimizing the number of assembly operations required to implement an operating system and/or enabling the volume occupied by the circuit to be reduced. It is an object of the present invention to mitigate this problem.